tests: Set basedir to None, not ''.

Turns out setting basedir='' creates cache/ in cwd.

tests/unit/config/test_config.py

@@ -316,7 +316,7 @@ class TestConfigInit:
     def test_config_none(self, monkeypatch):
         """Test initializing with config path set to None."""
         args = types.SimpleNamespace(confdir='', datadir='', cachedir='',
-                                     basedir='')
+                                     basedir=None)
         for k, v in self.env.items():
             monkeypatch.setenv(k, v)
         standarddir.init(args)